A heterojunction type photovoltaic device in which an amorphous silicon layer is stacked on a single crystalline or polycrystalline silicon substrate is disclosed in U.S. Pat. No. 4,496,788. This heterojunction type photovoltaic device has a low-cost advantage as compared to a photovoltaic device in which both of an n type layer and a p type layer are composed of single crystals, since its p-type layer and n type layer are formed of an amorphous silicon layer, which allows a low a temperature process to be employed. Furthermore, this heterojunction type photovoltaic device has an advantage that its conversion efficiency is high as compared to an amorphous photovoltaic device in which amorphous layers form a pin junction. Accordingly, such a heterojunction type photovoltaic device is in the spotlight as a low-cost photovoltaic device having an excellent conversion efficiency.
However, in a heterojunction type photovoltaic device in which an amorphous silicon layer is stacked on a silicon crystal, there has been a problem that a large number of interface states are produced between the crystalline layer and the amorphous layer so that a high conversion efficiency could not be obtained because of a recombination of carriers.
Also, there has been a problem that a considerable difference between the crystalline silicon band gap and the amorphous silicon band gap, produces a large barrier in the vicinity of the interface, so that transportation of the photoexcited carriers is prevented by this barrier, which does not allow a sufficient output of the photocurrent.
Furthermore, there also has been a problem that a large number of dangling-bonds exist in the vicinity of these interfaces because of lattice mismatch of the crystalline silicon and the amorphous silicon, whereby interface states are increased, resulting in an increase in the recombination of the carriers.
To sum up, in a conventional heterojunction solar battery in which an amorphous silicon semiconductor is stacked on a crystalline silicon semiconductor, there have been the problems that photo current could not be outputted sufficiently and the conversion efficiency was not satisfactory due to the large difference between band gaps and due to the lattice mismatch.